1. Field of the Invention
The present invention relates to a method for allocating a code to a reverse link in a multi-carrier code division multiple access system. The present invention relates in particular to a method for allocating a scrambling/spreading code to such a reverse link.
2. Description of the Related Art
In recent years, Multi-Carrier Code Division Multiple Access (MC-CDMA) has been receiving widespread interest for wireless broadband multimedia applications. Multi-Carrier Code Division Multiple Access (MC-CDMA) combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in the article entitled “Multicarrier CDMA in indoor wireless radio networks” which appeared in Proceedings of PIMRC'93, Vol. 1, pages 109-113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of Multicarrier CDMA” published in IEEE Communication Magazine, pages 126-133, December 1997.
Unlike DS-CDMA (Direct Spread Code Division Multiple Access), in which the signal of each user is multiplied in the time domain in order to spread its frequency spectrum, the signature here multiplies the signal in the frequency domain, each element of the signature multiplying the signal of a different sub-carrier.
MC-CDMA combines the advantageous features of CDMA and OFDM, i.e. high spectral efficiency, multiple access capabilities, robustness in presence of frequency selective channels, high flexibility, narrow-band interference rejection, simple one-tap equalisation, etc.
However MC-CDMA presents a significant drawback which is due to the multi-carrier modulation. Indeed, as shown below, an MC-CDMA signal consists in a sum of modulated sub-carriers which may result in a high dynamic range.
More specifically, FIG. 1 illustrates the structure of an MC-CDMA transmitter for a given user k. We consider here a reverse link, i.e. we suppose that the transmitter is located in the mobile terminal of the user. Let d(k)(n) be the symbol to be transmitted from user k at time nT to the base station, where d(k)(n) belongs to the modulation alphabet. The symbol d(k)(n) is first multiplied at 110 by the product of a spreading sequence, denoted c(k)(t), and a scrambling sequence specific to the user and denoted σ(k)(t). The spreading sequence consists of N “chips”, each “chip” being of duration Tc, the total duration of the spreading sequence corresponding to a symbol period T. Without loss of generality, we assume otherwise specified in the following that a single spreading sequence is allocated to the user. In general, a user may be allocated one or a plurality of orthogonal spreading sequences, according to the data rate required. In order to mitigate cellular interference (inter-cell interference and intra-cell interference), the spreading/scrambling sequences allocated to different users are preferably chosen orthogonal.
The results of the multiplication of the symbol d(k)(n) by the elements of the product sequence are multiplexed over a subset of frequencies of an OFDM multiplex. In general the number N of frequencies of said subset is a sub-multiple of the number L of frequencies of the OFDM multiplex. We denote Ωk the subset of {0, . . . , L−1} indexing the frequencies used by user k, cl(k), l∈Ωk the values of the corresponding spreading sequence elements σl(k), l∈Ωk the values of the scrambling sequence elements. The block of symbols multiplexed in 120 is then subjected to an inverse fast Fourier transformation (IFFT) in the module 130. In order to prevent intersymbol interference, a guard interval of length typically greater than the duration of the impulse response of the transmission channel, is added to the MC-CDMA symbol. This is achieved in practice by adding a prefix (denoted Δ) identical to the end of the said symbol. After being serialised in the parallel to serial converter 140, the MC-CDMA symbols are amplified in amplifier 150 in order to be transmitted over the reverse link transmission channel. The MC-CDMA method can therefore be analysed into a spreading in the spectral domain (before IFFT) followed by an OFDM modulation.
The signal Sk(t) at time t which is supplied to the amplifier before being transmitted over the reverse link transmission channel can therefore be written, if we omit the prefix:
                                          S            k                    ⁡                      (            t            )                          =                                                            d                                  (                  k                  )                                            ⁡                              (                n                )                                      ⁢                                          ∑                                  l                  ∈                                      Ω                    l                                                              ⁢                                                c                  l                                      (                    k                    )                                                  ⁢                                  σ                  l                                      (                    k                    )                                                  ⁢                                  exp                  ⁡                                      (                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                        l                                            ⁢                      t                                        )                                                  ⁢                                                                  ⁢                for                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                T                                              ≤          t          ≤                                    (                              n                +                1                            )                        ⁢            T                                              (        1        )            where fl, l=0, . . . ,L−1 are the frequencies of the OFDM multiplex.
The dynamic range of the MC-CDMA signal Sk(t) is estimated by the so-called Peak to Average Power Ratio (PAPR) expressed by:
                              PAPR          ⁡                      (                          S              k                        )                          =                                            max                              T                m                                      ⁢                          |                                                S                  k                                ⁡                                  (                  t                  )                                            ⁢                              |                2                                                                                        1                                  T                  m                                            ⁢                                                ∫                  0                                                  T                  m                                                      |                                          S                k                            ⁡                              (                t                )                                      ⁢                          |              2                        ⁢                          ⅆ              t                                                          (        2        )            where Tm is the time window over which the MC-CDMA signal is observed. Equivalently, the dynamic range of the MC-CDMA signal is expressed by the so-called Crest Factor (CF), simply defined as:CF(Sk)=√{square root over (PAPR(Sk))}  (3)An MC-CDMA signal of large PAPR is particularly sensitive to non-linearities of the output amplifier, also referred to as High Power Amplifier (HPA). Indeed, above a given signal amplitude, the HPA enters a saturation zone and the amplified signal is significantly distorted. The level of distortion of the amplified signal Ak(t) is expressed by the so-called Output Back-Off (OBO):
                              OBO          k                =                              P            sat                                E            ⁡                          (                              |                                                      A                    k                                    ⁡                                      (                    t                    )                                                  ⁢                                  |                  2                                            )                                                          (        4        )            where E(|Ak(t)|2) is the mean power of the amplified signal and Psat=Asat2 represents the saturation power of the amplifier where Asat is the amplitude saturation threshold. Equivalently, the level of distortion can be assessed at the input of the amplifier by the so-called Input Back-Off (IBO):
                              IBO          k                =                              E            ⁡                          (                              |                                                      S                    k                                    ⁡                                      (                    t                    )                                                  ⁢                                  |                  2                                            )                                            P            sat                                                        (                      4            ′                    )                    where E(|Sk(t)|2) is the mean power of the input signal.
The characteristics of an HPA amplifier is shown in FIG. 2 where I(t) and O(t) respectively denotes a signal amplitude at the input and the output of the amplifier. An example of MC-CDMA signal Sk(t) is also represented at the input of the amplifier. It should be noted that, by decreasing the output back-off, the operating point of the amplifier is shifted towards saturation and for a given OBO threshold non-linear distortion appears. The lower the output back-off, the higher the distortion but also the higher the amplifier efficiency. The output back-off has therefore to be adjusted to an optimal value in order to obtain the best efficiency for a given level of distortion.